This invention relates generally to resin coated permanent magnets and in particular, to a permanent magnet having a waterproof organic resin coating to provide superior oxidation resistance and strength.
Permanent magnets include ferrite magnets, alnico magnets and rare-earth magnets. The demand for rare-earth magnets has grown in proportion to the growing demand for smaller and higher efficiency electrical appliances for office automation such as computers, word processors and facsimile machines.
Rare-earth magnets are grouped into three classes by method of manufacture. These classes include sintered magnets, bonded magnets and cast magnets.
Typical rare-earth magnets are also grouped by composition. Specifically, rare-earth magnets include a rare-earth metal in combination with either cobalt or ferrite.
European Patent No. 108474 issued to General Motors Corp. discloses a rare-earth magnet including a rare-earth metal and iron which is obtained by a rapid quenching method. In the rapid quenching method, a ribbon-like material having a thickness of 20 .mu.m is provided. The ribbon-like material is an aggregate of crystals having a diameter between about 0.1 and 0.5 .mu.m, which is smaller than the critical diameter of a uniaxial particle. The material is pulverized to a particle size of less than about 177 .mu.m while maintaining coercive force and the pulverized material is used to form a resin bonded magnet.
Rare-earth magnets are further classified into two groups based on the coercive force mechanism of the magnet. One of the groups includes those rare-earth magnets which function in accordance with a 1-5 system magnetic model. These include rare-earth transition metal compounds having formulas such as SmCo.sub.5, CeCo.sub.5, Sm.sub.0.5 Ce.sub.0.5 Co.sub.5, YCo.sub.5, PrCo.sub.5 and Sm(CoCu).sub.5. Nuclear magnetic intermetallic compounds of at least one rare-earth metal and at least one transition metal including magnets based on R-Fe-B are also included in this group.
The second type of permanent magnets function in accordance with a planning model of 2-17 system magnets. These two-phase separate type or analysis hard type magnets include rare-earth transition metal intermetallic compounds having formulas such as: EQU Sm(Co.sub.bal Cu.sub.0.05 Fe.sub.0.02 Zr.sub.0.02).sub.8.0 EQU Sm(Co.sub.bal Cu.sub.0.06 Fe.sub.0.022 Ti.sub.0.016).sub.7.6 EQU Sm.sub.0.8 Y.sub.0.2 (Co.sub.bal Cu.sub.0.06 Fe.sub.0.20 Nb.sub.0.018).sub.7.8 EQU Sm.sub.0 7 Ce.sub.0.3 (Co.sub.bal Cu.sub.0.06 Fe.sub.0.24 Zr.sub.0.02).sub.7.8 and EQU Sm.sub.0 5 Pr.sub.0.5 (Co.sub.bal Fe.sub.0.3 Cu.sub.0.07 Zr.sub.0.02).sub.7.6.
The amount of cobalt is approximately 0.91. However, this amount is specified as a balance since a limited amount of impurities may be included.
Rare-earth transition metal intermetallic compounds including rare-earth metals, transition metals and semi-metals or semiconductor elements are reactive with oxygen. Specifically, the magnetic surface reacts with atmospheric oxygen to create rust. R-Fe-B magnets cause particular problems. When R-Fe-B magnets are incorporated into motors, relays and the like, oxides produced on the surface of the magnet are removed as the equipment operates and cause such significant problems in the equipment that the magnet itself is unsuitable for practical use.
European Patent No. 101552 issued to Sumitomo Tokushu Kinzoku Kabushiki Kaisha relates to rare-earth iron series permanent magnets obtained by a sintering method and consisting primarily of neodymium, iron and boron. However, the European patent does not recognize that rusting is a problem.
Japanese Patent Laid-Open Application No. 56-81908 discloses that rust can be prevented by coating a resin such as an epoxy resin on a rare-earth magnet. However, subtle pin-holes are generated in the plating or coating layer and it is difficult to prevent these pin-holes. As a result, rust occurs when water contacts the magnet through pin-holes in the coating layer.
The pin-holes are generated because the magnets do not have an entirely uniform planar or mirror surface. Rather, the rare-earth magnets have subtle uneven irregularities or spaces between magnetic particles. The resin can therefore not be coated uniformly.
Furthermore, solvent in the plating or coating solution is volatilized when the layer dries even when the layer is coated as uniformly as possible. Pin-holes occur at the volatilized portions. For these reasons, it is extremely difficult to provide a coating layer on a rare-earth magnet without generating pin-holes.
The generation of pin-holes is not a significant problem in prior art magnets such as Sm-Co magnets which include only a small amount of iron. In contrast, prior art magnets including a rare earth metal and a large amount of iron are apt to rust. When these magnets are incorporated into rotating machines such as motors, VCMs, speakers and relays to provide a magnetic circuit the rust which has been generated causes the magnetic performance to deteriorate for the reasons discussed.
Accordingly, it is desirable to provide an improved permanent magnet having superior corrosion and weathering resistance.